The present invention relates to an energy storage apparatus having at least two storage modules that are electrically connected in series. Each storage module has a multiplicity of storage cells that are electrically connected in series. The following discussion of related energy storage art is provided to assist the reader in understanding the advantages of the invention. It is not to be construed as an admission that this related art is prior art to this invention.
The storage modules for energy storage apparatus of this type have rechargeable batteries cells or double layer capacitors as storage cells. Rechargeable batteries and double layer capacitors are rechargeable charge stores which, in the charged state, can make electrical power available for a load. The voltage at which that power is made available is dependent on the type of charge store. In the case of lithium ion rechargeable batteries, that voltage is in the neighborhood of approximately 3.3V, and approximately 2.5V in the case of double layer capacitors. In order to supply loads that require higher voltages, it is known for a plurality of storage cells to be connected in series to form a storage cell bank. In this case, the supply voltage provided by such a storage cell apparatus corresponds to the sum of the individual cell voltages of the series-connected storage cells.
During the charging of the series-connected storage cells, it is unavoidable that voltage differences arise among the storage cells in a storage cell bank—designated hereinafter as a storage “module”—owing to capacitance tolerances. Thus, in order to optimally utilize the capability of the storage cells of a storage module for energy storage, a uniform or “balanced” voltage distribution among the storage cells of the storage module has to be achieved by charge equalization.
For an electric motor, such as that of a hybrid vehicle, for example, the required voltage exceeds the voltage of an individual storage module. In the event that the required supply voltage is significantly higher than the voltage of the storage module, a plurality of storage modules have to be electrically connected in series. This also applies to high-voltage apparatus such as trams. In those applications, in particular, a large number of storage modules are electrically connected in series, each storage module having a multiplicity of storage cells that are also electrically connected in series, in energy storage apparatus having m storage modules, it is necessary balance the m storage modules among one another, in addition to balancing the n storage cells in each of the m storage modules.
There are two types of methods for balancing of the storage cells in each storage module in an energy storage apparatus, namely dissipative and regenerative methods. In the dissipative methods, the storage cells in a storage module are discharged by part of their stored charge being converted into heat, in non-reactive resistors for example. In the regenerative methods, the excess energy or charge in individual storage cells of a storage module is redistributed to other storage cells of the storage module. Dissipative methods are disclosed in “Lithium Ion Battery Monitoring System AD7280” from Analog Devices, 2008, and in “Multicell Battery Stack Monitor LTC6802-1” from Linear Technology, 2009, for example.
Multiple storage modules in an energy storage apparatus can be balanced among one another by all n storage cells in each module being discharged simultaneously, so that stored energy is converted into heat. The disadvantage there is that a high level of thermal power is produced, and dissipating that thermal power produced in the individual storage modules poses problems.
Regenerative methods are subdivided into inductive methods and capacitive methods. Inductive methods are disclosed in “PowerLAN™ Dual-Cell Li-Ion Battery Monitor With PowerPump™ Cell Balancing” from Texas Instruments, 2009, and from DE 102008021090 A1, for example. Capacitive methods are disclosed in the EPE publication, Barcelona 2009, entitled “Analysis and Improvements of Novel Voltage Balancer for an Electric Double Layer Capacitor Employing a CW circuit”.
Balancing the m storage modules of an energy storage apparatus among one another by regenerative methods is only is possible if additional high-voltage cables are provided between the m storage modules and the terminals of the energy storage apparatus, which requires a considerable additional outlay. It would be desirable and advantageous to provide an improved storage apparatus that obviates prior art shortcomings by addressing this problem. Thus the invention provides energy storage apparatus having a plurality of storage modules in which an inductive regenerative balancing method can be employed without the use of additional high-voltage cables.